The present invention relates in general to processing of raw rubber, and in particular to a method of and an automatic device for forming predetermined charges of at least one sort of raw rubber to be fed in a mixer used in the manufacture of rubber articles such as motor car tires and the like.
In manufacturing rubber articles such as tires the raw rubber charges for the mixer have to be formed within relative narrow tolerances, for example, charges containing 300 kilograms of constituents have to be made with a tolerance of .+-.2 kilograms. Each of such charges is constituted in accordance with a given prescription mostly by several sorts of synthetic rubber whereby each of the sorts forming the constituents of the charge has to be also measured within prescribed tolerances. In addition, for the preparation of a charge there is available only very short time interval inasmuch as the latter depends on the cycle of the subsequent mixer which amounts between 2.5 to 3 minutes only.
In practice, the actual raw rubber material employed as a starting material for the formation of the charges has the shape of rectangular rubber blocks or bales each having approximately 700 to 800 millimeters in length, 350 to 500 millimeters in width and 100 to 200 millimeters in height and a weight between 30 to 35 kilograms depending on the output of synthetic rubber plants. To meet the mixing prescription the charges in practice can never be formed of a number of whole rubber bales but it is necessary because of the desired amount of rubber components according to individual sorts and because of variations in the weight of the assorted bales to feed into the mixer also fractions of the bales Under these circumstances, considerable difficulties arise in forming a charge within the limits of the available short time interval.
In a known method of forming rubber charges the measurement of the raw rubber constitutents is carried out in such a manner that firstly an amount of raw rubber bales is brought on a scale where the number of bales in the batch is completed to a value which approaches as closely as possible to the desired weight of the component part or of the whole charge, thereupon in dependence upon the magnitude of the weight difference a piece or a disc is cut off from a rubber bale and thus the weight of the constituted part or of the charge is brought to the desired value. In this known method, therefore, each weight difference has to be converted into a length of a rubber bale which is to be separated from an additional bale and added to the charge or to a constituent part of the charge. Due to the fact however that the cross-section of respective rubber bales is not uniform, firstly because of different shape of the bales and secondly because of different dimensions of respective bales, such known method does not frequently meet the required accuracy in the weight of the charge but results in more or less large tolerances. In order to diminish this shortcoming it has been already devised to cut out the complementary fraction of a bale from an intermediate section of the latter where the relation between the length of a bale and its weight is more constant. Because of the fact, however, that the shape variations of respective rubber bales occur with equal frequency both with respect to the height and width and with respect to the length, this proposed measure brings about no general improvement in the accuracy of adjustment of desired weight of each charge or of its component parts.
From the German published patent application No. 28 39 53 a machine is known for performing the prior art charge forming method which includes a group of a first and of a second layer for feeding rubber bales or bale pieces to a first and to a second cutting device, and further includes a discharging conveyor which feeds pieces cut off from the cutting devices through an automatic scale; the first conveyor is arranged in a fixed position relative to the first cutting device whereas the second conveyor is movable relative to the second cutting device and the discharging conveyor is controlled by the automatic scale. This known machine, however, has also the disadvantages of the aforedescribed known method.